1. Field of the Invention
The present invention relates to a technology that, in a vehicle driving apparatus including an internal combustion engine and an electric motor, reduces confined noise caused by variation of the torque outputted from the internal combustion engine.
2. Description of Related Art
Vehicle driving apparatuses are known which include a differential mechanism that distributes engine output to a first motor and an output shaft, and a second motor that is arranged between the output shaft of the differential mechanism and the drive wheels. An example of a vehicle driving apparatus is provided by the hybrid vehicle driving apparatus disclosed in Patent Document 1 (Japanese Patent Laid-Open Publication No. 2003-127679). In this type of hybrid vehicle driving apparatus, a differential mechanism is composed of a planetary gear device, for example, which mechanically transfers a main part of the power from the engine to the drive wheels by differential action.
The rest of the power from the engine is electrically transferred from the first motor to the second motor via an electrical path. Accordingly, the vehicle driving apparatus serves as a transmission that allows the differential mechanism to electrically change its transmission ratio, just as an electrical continuously variable transmission does. As a result, a control device controls the vehicle driving apparatus to drive the vehicle while keeping the engine in an optimal operating state, thereby improving fuel efficiency.
Generally, various types of vibration and noise are generated in vehicles. For example, various types of vibration and noise are generated by periodic cylinder ignition (explosion) in an engine, and the rotational variation caused by variation of engine torque in the reciprocating motion of pistons, both acting as a forcing source (also acting as a vibration source or forcedly vibrating force). Vibration is transmitted to other parts and systems of a vehicle, such as an engine suspension system, an exhaust pipe system, and a bodywork system. The engine suspension system is composed of a power mount and an engine mount. The power mount couples an engine to a transmission (or transaxle), and the like. The transmitted vibration is amplified by the phenomenon of resonance in the systems of the vehicle. Thus, vibration and confined noise are generated in individual sections of the vehicle.
It is well known that the phenomenon of resonance occurs within a specific engine rotational speed range as a resonance range. For example, the resonance range may correspond to a specific engine rotational speed range where engine rotational speed is less than idle rotational speed. When the rotational speed of the engine increases upon engine starting, the rotational speed of the engine will reach that specific engine rotational speed range. In this case, the resonance phenomenon may occur.
Reducing confined noise generated in this way has been tried for improving comfort and quietness in vehicles, but the standards required have been set higher in recent years. In hybrid vehicles, any change in the characteristics of a damper is limited by its specifications, for example. Reduction of such confined noise by hardware is limited.
Another technology is disclosed in Patent Document 2 (Japanese Patent Laid-Open Publication No. 2005-199971) which suppresses confined noise. In the technology, in relation to operation lines of an internal-combustion engine, that is, lines that represent the relation between target rotational speed and target torque as target operation points, in addition to an operation line for fuel consumption to provide good fuel efficiency, an operation line for confined noise suppression to suppress confined noise is provided. If confined noise is generated, the internal combustion engine is operated with the switched operation line for confined noise suppression, alternatively a motor provides compensation torque that vibrates in opposite phase to the phase of torque generated by the vibration associated with the rotational speed of an engine.
However, in the method of the Patent Document 2, the following problems arise. In the switched operation line for confined noise suppression, the internal combustion engine is operated without consideration for fuel efficiency. In addition to this, outputting the compensation torque in opposite phase consumes battery power, arising energy loss. For this reason, fuel efficiency is reduced. Furthermore, in the case where a battery has limited capacity to receive and provide power under low temperature, for example, this method is not feasible.